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How is boiling different from evaporation?

  1. Aug 26, 2009 #1
    Thinking about this, I would say that there really is no difference. When water evaporates, surface molecules gain enough kinetic energy to change state (if it even makes sense to talk about state in the thermodynamic sense when dealing with discrete molecules). During boiling, you are giving all the molecules, including those in the bulk (i.e., below the surface), enough energy to change state. Is this accurate? If so, how does this jive with models of liquids and gases being continuous substances?
     
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  3. Aug 26, 2009 #2
    Actually I'm not sure that you're capable of understanding the answer (no offense), but to understand this you have to have some background about statistical mechanics.

    About discrete levels, you don't have discrete levels anymore when you're dealing with this large number of molecules, the energy spacing of the system is proportional to e^-N, where N is the number of particles. There you deal with what's called the canonical ensemble and you use Van der Waals equation of state to describe this phase transition. I have been discussing in this forum this problem but in more advanced form, the link is the following.

    https://www.physicsforums.com/showthread.php?t=331652

    The issue is that you need to sustain positive compressibility and continuous Free energy of the system, and the solution is constant temperature and increasing energy.

    Simplifying the problem, I'll tell you that the molecules on the surface abandon the liquid faster than others because they face less pressure due to the fact they're on the surface.

    I don't know if this answers, but you can ask me another question so we can discuss more about this, and so I can convince you :)

    Good luck :)
     
  4. Aug 26, 2009 #3

    Mapes

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    You've got it. Boiling occurs when the gas phase is more stable than the liquid phase. Even if the liquid phase is more stable, though, evaporation will still tend to produce gas until the condensation rate matches the evaporation rate. The reason is, as you said, some of the surface atoms/molecules have enough energy to leave the liquid.

    Boiling and evaporation both involve a liquid->gas phase change, but boiling requires the vapor pressure to be at least at high as the surrounding pressure. That's why boiling is also characterized by bubbles nucleating within the liquid.
     
  5. Aug 26, 2009 #4

    Andy Resnick

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    Minor quibble- boiling is different than evaporation because boiling involves nucleation *at a fluid-solid interface* while evaporation is conceptually similar to diffusion of one fluid into another.

    nucleation of the gas phase does not occur within the bulk fluid; that's how we can make supercooled solutions reasonably easily- by using a very clean, very smooth, container.
     
  6. Aug 26, 2009 #5
    Minor quibble over your minor quibble. Boiling amost invariable involves hot spots. OK, it's not really a quibble but a question. The most uniform heating I can imagine without nucleation sites would be a smooth container of liquid in a microwave oven. Do you think we would see spontaneous bubbles forming in the bulk of the liquid?
     
  7. Aug 26, 2009 #6

    russ_watters

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    Huh, I was thinking of the exact same thing with the same example. If you have a pot on a stove, naturally all the bubbles form on the bottom of the pot where the metal gets hot. In a metal pot in a microwave, the only place for the microwaves to "get in" would be on the surface, so I would think you'd only see the bubbles forming at the surface and there'd be no "rolling" boil.
     
  8. Aug 27, 2009 #7

    Danger

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    Wouldn't that just wreck the oven? :confused:
     
  9. Aug 27, 2009 #8

    Andy Resnick

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    Nucleation in the bulk phase can happen, but the energy barrier is significantly higher than that at an inhomogeneity at the solid-fluid interface. So bubbles nucleate at walls. If the surface is highly homogeneous, when nucleation happens in the bulk, it happens (nearly) *everywhere at the same time*, which leads to a huge mess.

    I'm sure there's youtube videos of this.
     
  10. Aug 27, 2009 #9
    In fact boiling can be explained much more simply. When a liquid boils it does so because the partial pressure of vapour in the atmosphere above equals the pressure of the other gases. When this occurs boiling happens. When the vapour pressure is below the pressure of the other gasses above the liquid evaporation happens.

    Bubbling is not defination of boiling. If their where no nucleation sites for the bubbles to form boiling would still happen. Remeber that boiling is a phases transition. The temperature of the liquid remains constant when this happens and cannot rise until it is all gone. This is the other difference between boiling and evaporation.
     
  11. Aug 27, 2009 #10
    I don't find a definition of boiling without (vapor) bubbles anywhere on the internet. References? Without nucleation sites, boiling is triggered throughout a volume somewhere above the phase transition temperature (apparently*), as Andy Resnick points out, and these are bubbles. I don't think surface molecules separating from the surface as gasious molecules going on at a temprature above the phase transition temperature would be called boiling.
     
  12. Aug 27, 2009 #11
    (apparently*) What determines the maxium temperature for a superheated liquid?
     
  13. Aug 28, 2009 #12

    Andy Resnick

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    I'm not sure there is a maximum, if I understand the spirit of your question. Unless there is a perturbation- and the magnitude of the required perturbation surely decreases until it's comparable to thermal noise- the material will remain in its metastable state until such a perturbation occurs.

    L&L volume 10 has a chapter on this, and I should point out that the liquid-gas phase change is a *first order* phase change. Second order phase changes do not require nucleation as there is no energy barrier (latent heat) to the phase change.
     
  14. Aug 28, 2009 #13

    Mapes

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    A quibble in return: :smile: The energy barrier associated with nucleation isn't the latent heat; rather, it's related to the penalty for creating new surface area at the boundary of the new phase. Spinodal decomposition, for example, involves a latent heat but no nucleation.
     
  15. Aug 28, 2009 #14

    russ_watters

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    Perhaps it depends on the metal, but I've seen aluminum foil in a microwave do nothing but shield what was inside.
     
  16. Aug 29, 2009 #15

    GT1

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    As far as I know bubbles are form on walls because of the surface roughness. On the wall's cavities nucleuses with smaller volume have larger radius - the Laplace pressure is lower and the nucleus is more stable - the bubbles form inside the wall's cavities.
     
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